Is Code 1893 Part 2 2016 Pdf

[Kennedy Tadder]

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The concept of stiffness modifiers is introduced for the first time in IS 1893 (Part 1) : 2016. The clause no. 6.4.3.1 of the code defines requirements for structural analysis. It is mentioned in the clause that for structural analysis, we should consider reduced moment of inertial for RCC structures. For columns, 70 percent of Igross should be considered and for beams, 35 percent of Igross to be considered. This clause has generated many questions among the group of structural engineers. The overall objective of writing this article is to collate views/suggestions from the wider group of engineers. In the following section, I have attempted to give answers to few questions, received from different engineers. The answers are given for buildings having height less than 50 m. The comparison of stiffness modifiers with IS 16700 : 2017 & IS 15988 : 2013 will be done in the subsequent article.

Before this clause, in the structural analysis, we were considering 100% of moment of inertia for RCC beams and columns. In RCC member, the cracks will generate in the tension zone of concrete due to application of different loads. Due to these cracks, the moment of inertia of RCC member will be lesser than the gross moment of inertia. Hence, to account for the reduced moment of inertia of the cracked section, the concept of stiffness modifiers is introduced in the code.

The pattern and extent of cracks will vary substantially from structure to structures and members to members even in a similar structure. It is very difficult to assign the unique values of the stiffness modifiers for different members. The values which are suggested in the code are based on the numerous experiments and might have been referred from different international standards. The stiffness modifier value for column is higher than the beam since the columns will have higher axial compression than the beam. Hence, the extent of cracks would be lesser in the columns as compared to the beams. Hence, the stiffness modifier value for column is higher than the beams.

1) Since we are considering the reduced moment of inertia, the overall stiffness of the structure will reduce. Due to the reduced stiffness, the structure will be relatively flexible and hence would attract the lower seismic forces.

The stiffness modifiers should be considered only for the structural analysis. The analysis results to be derived considering the stiffness modifiers. The structural design to be done with the conventional procedure considering the analysis results.

Generally, the stiffness modifiers are different for serviceability and the ultimate conditions. As discussed above, the stiffness modifiers defined in IS 1893(Part 1) : 2016 are for the ultimate condition. If we consider the same stiffness modifiers for the serviceability condition as well then the moment at beam column junction will be higher and the span moment will be lesser as compared to the model without stiffness modifiers. In my opinion, the span moment may err on the unconservative side, if we consider the same stiffness modifiers for the serviceability condition.

As mentioned above, the stiffness modifiers to be considered only for seismic load case. The structural analysis is to be performed with the seismic loads having stiffness modifiers. Thereafter, the analysis results should be used for design of the foundation.

Consideration of the stiffness modifiers will reduce the seismic demand on the structure. But at the same time, it will result in relatively higher drifts due to increased flexibility of the structure. The stiffness modifiers value should be different for the serviceability and the ultimate conditions. The stiffness modifiers are to be used only for structural analysis. The comparison of stiffness modifiers with IS 16700 : 2017 & IS 15988 : 2013 will be done in the subsequent article. The above mentioned answers are based on my understanding. If you have further questions / difference of opinion, then please share details in the below comment box.

The Indian subcontinent has a history of devastating earthquakes.[1] The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year.[2] Geographical statistics of India show that almost 58% of the land is vulnerable to earthquakes. A World Bank and United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050.[3] The latest version of seismic zoning map of India given in the earthquake resistant design code of India [IS 1893 (Part 1) 2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version, which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.

The National Center for Seismology Ministry of Earth Sciences is a nodal agency of the Government of India dealing with various activities in the fields of seismology and allied disciplines. The major activities currently being pursued by the National Center for Seismology include a) earthquake monitoring on a 24/7 basis, including real time seismic monitoring for early warning of tsunamis, b) operation and maintenance of national seismological network and local networks, c) seismological data centre and information services, d) seismic hazard and risk related studies, e) field studies for aftershock/swarm monitoring and site response studies and f) earthquake processes and modelling.[4]

The MSK (Medvedev-Sponheuer-Karnik) intensity broadly associated with the various seismic zones is VI (or less), VII, VIII and IX (and above) for Zones 2, 3, 4 and 5, respectively, corresponding to Maximum Considered Earthquake (MCE). The IS code follows a dual design philosophy: (a) under low probability or extreme earthquake events (MCE) the structure damage should not result in total collapse, and (b) under more frequently occurring earthquake events, the structure should suffer only minor or moderate structural damage. The specifications given in the design code (IS 1893: 2002) are not based on detailed assessment of maximum ground acceleration in each zone using a deterministic or probabilistic approach. Instead, each zone factor represents the effective period peak ground accelerations that may be generated during the maximum considered earthquake ground motion in that zone.

Zone 5 covers the areas with the highest risk of suffering earthquakes of intensity MSK IX or more significantly. The IS code assigns a zone factor of 0.36 for Zone 5. Structural designers use this factor for the earthquake-resistant design of structures in Zone 5. The zone factor of 0.36 (the maximum horizontal acceleration that a structure can experience) is indicative of effective (zero period) level earthquakes in this zone. It is referred to as the Very High Damage Risk Zone. The regions of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the Rann of Kutch and the Andaman and Nicobar group of islands fall in this zone.Generally, the areas having trap rock or basaltic rock are prone to earthquakes.

This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. The IS code assigns a zone factor of 0.24 for Zone 4. Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, parts of the Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar Pradesh, Terai, a major portion of Bihar, North Bengal, the Sundarbans) and the capital of the country Delhi fall in Zone 4.In Maharashtra, the Patan area (Koynanagar) is also in Zone 4.

This zone is classified as a Moderate Damage Risk Zone which is liable to MSK VII. The IS code assigns a zone factor of 0.16 for Zone 3. Several megacities like Chennai, Mumbai, Pune, Kolkata,Bhubaneswar,Jamshedpur, Ahmedabad, Surat, Lucknow, Vadodara, Mangalore, Vijayawada, Coimbatore and the entire state of Kerala lie in this zone.

This region is liable to MSK VI or lower and is classified as the Low Damage Risk Zone. The IS code assigns a zone factor of 0.10 for Zone 2. It is the zone with low chances of having earthquakes. Cities like Bangalore, Hyderabad, Visakhapatnam, Nagpur, Raipur, Gwalior, Jaipur, Tiruchirappalli and Madurai are in this zone.

Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classified as Zone 1. But it is said[who?] that some areas considered safe are from stable landmass covered under the deccan plateau, those considered as zone 1.

Thank you all for actively participating in the live technical discussion related to draft IS 1893 (Part 1) on 18-MAY-23. If you have missed out the session, then you may go through the recording at following link:

He is passionate about Engineering profession with two decades of experience. He is having a dream for enhancing the engineering profession in different organisations. He completed graduation in Civil Engineering and Masters in Structures from Sardar Patel University. He is having unique experience of working in the specialized firm of civil / structural consultancy which grew as multidisciplinary firm (VMS), large multidisciplinary firm (L&T Chiyoda Ltd.) and owner based engineering set up (Adani Infra (I) Ltd.). He worked in different organisations at different levels, starting from junior design engineer to CEO. He is Founder & CEO of SQVe Consultants. He is pursuing Ph.D. in Structural Engineering related to earthquake resistant design of industrial steel structures.

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